The embodiments described herein relate generally to a systems and methods used in conjunction with an ultrasonic ablation device and, more specifically, to systems and methods of controlling the delivery of ultrasonic energy to a bodily tissue via a transmission member (e.g., a catheter, a probe or the like).
Known ultrasonic energy transmission systems are used in many different medical applications, such as, for example, in medical imaging, to disrupt obstructions and/or ablate bodily tissue. In known ultrasonic energy transmission systems for tissue ablation, ultrasonic energy is transferred from an ultrasonic energy source through a transducer horn and then a transmission member, such as a wire, to a distal head. Ultrasonic energy propagates through the transmission member as a periodic wave thereby causing the distal head to vibrate. Such vibrational energy can be used to ablate or otherwise disrupt bodily tissue, for example, a vascular obstruction, a kidney stone or the like. To effectively reach various sites for treatment of intravascular occlusions or regions within the urinary tract, such ultrasonic transmission members are often constructed from a thin, flexible material, and have lengths of about 65 cm or longer.
Known ultrasonic energy transmission systems include a generator, a transducer assembly and a probe (or transmission member). The generator is configured to generate, control, amplify, and/or transfer an alternating electronic signal of a desired frequency (e.g., a voltage signal) to the transducer assembly. The transducer assembly typically contains one or more piezoelectric crystals, which, when excited by the high frequency electronic signal, expand and contract at high frequency. These high-frequency vibrations are amplified by the ultrasonic horn into the ultrasonic energy that is transmitted to a probe (or transmission member). The ultrasonic energy is transmitted to the distal end of the probe to ablate and/or otherwise disrupt a bodily tissue.
Because known probes are often traversed through tortuous anatomic structures to reach the site of treatment, transmission members are often constructed to be elastic and/or flexible, but also with sufficient strength to transmit ultrasonic energy to the distal tip (e.g., to ablate vascular or urinary obstructions). To find a balance between strength and flexibility, some known transmission members are tapered such that the diameter of the distal end portion decreases, thereby providing a transmission member having greater flexibility. For example, some known transmission members have a diameter at the proximal end that is greater than a diameter at a distal end. Moreover, some known transmission members can include a distal tip or “head” that is welded to the reduced diameter section and is positioned adjacent the tissue to be treated.
To maximize energy transmission to the target bodily tissue, known systems are often configured to produce ultrasonic energy having a frequency that matches the natural frequency of the energy delivery assembly (i.e., the transducer and/or probe assembly). When operating at the natural frequency (i.e., at resonance conditions), the amplitude of the ultrasonic energy wave (or signal) travelling through the transmission member is at its maximum. The transmission member can be thought of as having a standing wave of ultrasonic energy traveling along its length. More particularly, the standing wave produces a series of nodes (regions of minimum displacement) and anti-nodes (regions of maximum displacement) along the length of the transmission member. Thus, when operating in resonance conditions, the displacement and/or vibration at the anti-nodes are at a maximum for a given power level. Each of the anti-nodes can produce cavitations in fluids in contact with the transmission member to cause the destruction of the adjacent tissue.
Some known systems include algorithms to tune the frequency of the electronic signal (sent from the generator to the transducer) to more closely match the natural frequency of the energy delivery assembly. For example, in some known systems, the amplitude and/or frequency of the ultrasonic energy can be controlled by monitoring the current input (or electronic signal input) to the transducer assembly, and varying the power input to the transducer assembly to maintain the electrical current input to the transducer assembly at a constant level. Such known algorithms can be used to account for variations in the natural frequency of the transmission member due to part-to-part variation, and the conditions in which the energy delivery assembly is used, and the like.
Operating a known ultrasonic transmission member continuously at its natural frequency, however, can reduce the reliability of the transmission member. More particularly, operation at a single frequency produces standing waves in the transmission member that are substantially constant. Similarly stated, when operating at a constant frequency, the location of the vibration anti-nodes within the transmission member remains substantially constant. Thus, the continued application of high stress to such regions increases the likelihood of mechanical failure. Moreover, operation at a constant frequency, even at resonance, may not effectively ablate the target bodily tissue.
Thus, a need exists for an improved systems and methods of controlling the delivery of ultrasonic energy to a bodily tissue.